Kotlin, Start? Start! ֢അࢳ (pluu) Android Developer GDG Korea Android Organizer 

Agenda ● Kotlin Overview ● Kotlin द੘द ঌݶ જ਷ Ѫ ● Ӓېࢲ?? 

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Basic fun main(args: Array): Unit { println("Hello, world!") } 

Basic fun main(args: Array): Unit { println("Hello, world!") } Function Keyword 

Basic fun main(args: Array): Unit { println("Hello, world!") } Function Name 

Basic fun main(args: Array): Unit { println("Hello, world!") } Parameter Name 

Basic fun main(args: Array): Unit { println("Hello, world!") } Parameter Type 

Basic fun main(args: Array): Unit { println("Hello, world!") } Return Type 

Basic fun main(args: Array): Unit { println("Hello, world!") } Function Definition 

Basic fun main(args: Array): Unit { println("Hello, world!") } Unit return type can be omitted 

Basic fun main(args: Array) { println("Hello, world!") } 

Null Safe val text: String = null val text: String = null // Error, non-null Type 

val text: String? = null Null Safe 

val text: String? = null Nullable Keyword Null Safe 

val text: String? = null text = 100.toString() val text: String? = null text = 100.toString() // Error, Immutable Data Null Safe 

val text: String? = null text = 100.toString() Immutable Keyword Null Safe 

var text: String? = null text = 100.toString() Mutable Keyword Null Safe 

Null Safe if (data != null && data.value != null) { data.value.toString() } 

Null Safe data?.value?.toString() 

Control Flow if / when / try 

Control Flow statement expression No Return Value Return Value 

int a = 10; String value; if (a > 10) { value = "Over 10"; } else { value = "Under 10"; } val a = 10 val value: String if (a > 10) { value = "Over 10" } else { value = "Under 10" } Expression ( if / when / try ) Java Kotlin 

int a = 10; String value; if (a > 10) { value = "Over 10"; } else { value = "Under 10"; } val a = 10 val value: String if (a > 10) { value = "Over 10" } else { value = "Under 10" } Expression ( if / when / try ) Java Kotlin 

Expression ( if / when / try ) Java Kotlin val a = 10 val value = if (a > 10) { "Over 10" } else { "Under 10" } int a = 10; String value; if (a > 10) { value = "Over 10"; } else { value = "Under 10"; } 

Expression ( if / when / try ) Java Kotlin val a = 10 val value = if (a > 10) "Over 10" else "Under 10" int a = 10; String value; if (a > 10) { value = "Over 10"; } else { value = "Under 10"; } 

Expression ( if / when / try ) val a = 10 val value = if (a > 10) "Over 10" else "Under 10" Kotlin 

Expression ( if / when / try ) Java Kotlin val value: String if (a === 1) { value = "one" } else if (a === 2) { value = "two" } else if (a === 3) { value = "three" } else { value = "another" } String value; if (a == 1) { value = "one"; } else if (a == 2) { value = "two"; } else if (a == 3) { value = "three"; } else { value = "another"; } 

Expression ( if / when / try ) Java Kotlin val value: String when { a === 1 -> value = "one" a === 2 -> value = "two" a === 3 -> value = "three" else -> value = "another" } String value; if (a == 1) { value = "one"; } else if (a == 2) { value = "two"; } else if (a == 3) { value = "three"; } else { value = "another"; } 

Expression ( if / when / try ) Java Kotlin val value = when(a) { 1 -> "one" 2 -> "two" 3 -> "three" else -> "another" } String value; if (a == 1) { value = "one"; } else if (a == 2) { value = "two"; } else if (a == 3) { value = "three"; } else { value = "another"; } 

when (x) { parseInt(s) -> print("s encodes x") in 1..10 -> print("x is in the range") in validNumbers -> print("x is valid") !in 10..20 -> print("x is outside the range") else -> print("none of the above") } Expression ( if / when / try ) Kotlin 

when (x) { parseInt(s) -> print("s encodes x") in 1..10 -> print("x is in the range") in validNumbers -> print("x is valid") !in 10..20 -> print("x is outside the range") else -> print("none of the above") } Expression ( if / when / try ) Kotlin Expression 

when (x) { parseInt(s) -> print("s encodes x") in 1..10 -> print("x is in the range") in validNumbers -> print("x is valid") !in 10..20 -> print("x is outside the range") else -> print("none of the above") } Expression ( if / when / try ) Kotlin Range 

when (x) { parseInt(s) -> print("s encodes x") in 1..10 -> print("x is in the range") in validNumbers -> print("x is valid") !in 10..20 -> print("x is outside the range") else -> print("none of the above") } Expression ( if / when / try ) Kotlin Not In Range 

when (x) { parseInt(s) -> print("s encodes x") in 1..10 -> print("x is in the range") in validNumbers -> print("x is valid") !in 10..20 -> print("x is outside the range") else -> print("none of the above") } Expression ( if / when / try ) Kotlin Optional, statement / expression 

Default Parameter public String foo(String name, int number, boolean toUpperCase) { return (toUpperCase ? name.toUpperCase() : name) + number; } public String foo(String name, int number) { return foo(name, number, false); } public String foo(String name, boolean toUpperCase) { return foo(name, 42, toUpperCase); } public String foo(String name) { return foo(name, 42); } Java 

Default Parameter public String foo(String name, int number, boolean toUpperCase) { return (toUpperCase ? name.toUpperCase() : name) + number; } public String foo(String name, int number) { return foo(name, number, false); } public String foo(String name, boolean toUpperCase) { return foo(name, 42, toUpperCase); } public String foo(String name) { return foo(name, 42); } Java Default : false Default : 42 

Default Parameter fun foo(name: String, number: Int = 42, toUpperCase: Boolean = false) = (if (toUpperCase) name.toUpperCase() else name) + number 

Default Parameter fun foo(name: String, number: Int = 42, toUpperCase: Boolean = false) = (if (toUpperCase) name.toUpperCase() else name) + number 

Example ● setter / getter ● hashCode ● equals ● toString class Item { private String type1; private int type2; private float type3; private float type4; } 

Example class Item { private String type1; private int type2; private float type3; private float type4; public Item(String type1, int type2, float type3, float type4) { this.type1 = type1; this.type2 = type2; this.type3 = type3; this.type4 = type4; } public String getType1() { return type1; 

class Item { private String type1; private int type2; private float type3; private float type4; public Item(String type1, int type2, float type3, float type4) { this.type1 = type1; this.type2 = type2; this.type3 = type3; this.type4 = type4; } public String getType1() { return type1; } public void setType1(String type1) { this.type1 = type1; } public int getType2() { return type2; } public void setType2(int type2) { this.type2 = type2; } public float getType3() { return type3; } public void setType3(float type3) { this.type3 = type3; } public float getType4() { return type4; } public void setType4(float type4) { this.type4 = type4; } @Override public boolean equals(Object o) { if (this == o) return true; if (o == null || getClass() != o.getClass()) return false; Item item = (Item) o; if (type2 != item.type2) return false; if (Float.compare(item.type3, type3) != 0) return false; if (Float.compare(item.type4, type4) != 0) return false; return type1 != null ? type1.equals(item.type1) : item.type1 == null; } @Override public int hashCode() { int result = type1 != null ? type1.hashCode() : 0; result = 31 * result + type2; result = 31 * result + (type3 != +0.0f ? Float.floatToIntBits(type3) : 0); result = 31 * result + (type4 != +0.0f ? Float.floatToIntBits(type4) : 0); return result; } @Override public String toString() { return "Item{" + "type1='" + type1 + '\'' + ", type2=" + type2 + ", type3=" + type3 + ", type4=" + type4 + '}'; } } Field ࣻ ߸҃द? ߸҃ ೦ݾ ● setter / getter ● hashCode ● equals ● toString 

Data Class data class Item( var type1: String?, var type2: Int, var type3: Float, var type4: Float) 

Data Class data class Item( var type1: String?, var type2: Int, var type3: Float, var type4: Float) data Keyword 

Data Class ● equals() / hashCode() ● toString() ● componentN() functions ● copy() // Destructuring Declarations val jane = User("Jane", 35) val (name, age) = jane println("$name, $age years of age") // prints "Jane, 35 years of age" 

Singleton Class public class A { private static final A instance = new A(); private A() {} public static A getInstance() { return instance; } public void b() { System.out.println("Singleton"); } } Java Custom Method 

Object Class object A { fun b() { println("Singleton") } } // Use A.b() 

Object Class object A { fun b() { println("Singleton") } } Object declarations 

Type Inference val s: String = "Value" 

// String val c = 'V' // Char val i = 10 // Int val l = 10L // Long val f = 10F // Float val d = 10.0 // Double val b = true // Boolean val sb = StringBuilder() // StringBuilder val s = "Value" Type Inference 

Smart Casts if (obj is String) { print(obj.length) } https://kotlinlang.org/docs/reference/typecasts.html#smart-casts 

Smart Casts if (obj is String) { print(obj.length) } Type Check 

Smart Casts if (obj is String) { print(obj.length) } Smart Casts 

Smart Casts when (x) { is Int -> print(x + 1) is String -> print(x.length + 1) is IntArray -> print(x.sum()) } 

Smart Casts when (x) { is Int -> print(x + 1) is String -> print(x.length + 1) is IntArray -> print(x.sum()) } Type Check 

Smart Casts when (x) { is Int -> print(x + 1) is String -> print(x.length + 1) is IntArray -> print(x.sum()) } Smart Casts 

Smart Casts when (x) { is Int -> print(x + 1) is String -> print(x.length + 1) is IntArray -> print(x.sum()) } Type Check 

Smart Casts when (x) { is Int -> print(x + 1) is String -> print(x.length + 1) is IntArray -> print(x.sum()) } Smart Casts 

Smart Casts when (x) { is Int -> print(x + 1) is String -> print(x.length + 1) is IntArray -> print(x.sum()) } Type Check 

Smart Casts when (x) { is Int -> print(x + 1) is String -> print(x.length + 1) is IntArray -> print(x.sum()) } Smart Casts 

Extension Function inline fun Fragment.toast(message: CharSequence): Unit = activity.toast(message) fun Context.toast(message: CharSequence) = Toast.makeText(this, message, Toast.LENGTH_SHORT).show() https://github.com/Kotlin/anko/blob/d5a526512b48c5cd2e3b8f6ff14b153c2337aa22/anko/library/static/ commons/src/dialogs/Toasts.kt // Use in Activity/Fragment toast("Hello World") 

Extension Function fun Context.toast(message: CharSequence) = Toast.makeText(this, message, Toast.LENGTH_SHORT).show() 

Extension Function fun Context.toast(message: CharSequence) = Toast.makeText(this, message, Toast.LENGTH_SHORT).show() Receiver Type 

Extension Function fun Context.toast(message: CharSequence) = Toast.makeText(this, message, Toast.LENGTH_SHORT).show() Function Definition 

Extension Function fun Context.toast(message: CharSequence) = Toast.makeText(this, message, Toast.LENGTH_SHORT).show() Function Body 

Higher-Order Functions and Lambdas fun List.map(transform: (T) -> R): List { val result = arrayListOf() for (item in this) result.add(transform(item)) return result } 

Higher-Order Functions and Lambdas fun List.map(transform: (T) -> R): List { val result = arrayListOf() for (item in this) result.add(transform(item)) return result } Function Type 

Higher-Order Functions and Lambdas val list = listOf(1, 2, 3) list.map({ x: Int -> x * 2 }) 

Higher-Order Functions and Lambdas val list = listOf(1, 2, 3) list.map({ x: Int -> x * 2 }) list.map({ x -> x * 2 }) 

Higher-Order Functions and Lambdas val list = listOf(1, 2, 3) list.map({ x: Int -> x * 2 }) list.map({ x -> x * 2 }) list.map() { x -> x * 2 } 

Higher-Order Functions and Lambdas val list = listOf(1, 2, 3) list.map({ x: Int -> x * 2 }) list.map({ x -> x * 2 }) list.map() { x -> x * 2 } list.map { x -> x * 2 } 

Higher-Order Functions and Lambdas val list = listOf(1, 2, 3) list.map({ x: Int -> x * 2 }) list.map({ x -> x * 2 }) list.map() { x -> x * 2 } list.map { x -> x * 2 } list.map { it * 2 } 

൞ݎಞ ੺ݎಞ 

੺ݎಞ 1. Kotlin? ● ࢜۽਍ ঱যۄח Ѣࠗх / ۞׬ ழ࠳ / ࠛউх / ৈ۞ ੄ޙٜ … ● Functional Programming ● Hybrid Language о о૑ח ޙઁٜ ● ҡଳ਷ Best Practices ࠗ੤ 

੺ݎಞ 2. ೠӖ … ೠӖ … ೠӖ … ● ݆੉ হযਃ ● ೠӖ۽ ػ ޙࢲ ● ೠӖ۽ ػ ଼ 

੺ݎಞ 

੺ݎಞ 3. Function Programming ● ೣࣻ ӝ߈ ೐۽Ӓې߁ ӝߨ ● ೣࣻب ё୓ ● FP ҙ۲ ૑ध੉ ೙ਃೣ (ex, Predicate, High-Order Function) 

੺ݎಞ 3. FP ౱ ݯߡо ୶ୌ೧઼णפ׮. ੺؀ റਗ ইש 

੺ݎಞ 3. FP ౱ ݯߡо ୶ୌ೧઼णפ׮. ੺؀ റਗ ইש Lambda Clojure High-Order Function Reduce Dispatch Groovy Scala Currying Pure Function Monad Lazy 

੺ݎಞ 4. Overhead ● Kotlin Compiler ী ੄೧ ࢤࢿغח ௏٘ ● ࢤпೞח Ѫࠁ׮ ؊ ݆਷ ௏٘о ࢤࢿ ms ױਤ ର੉ ೙ਃदী ٜ݅যঠೞח ௏٘ Javaࠁ׮ וܽѪ਷ ಂ౟ 

੺ݎಞ 4. Overhead fun test(x: Any) { if (x is String) { println(x.length) println(x.length) println(x.length) println(x.length) } } public static final void test(@NotNull Object x) { Intrinsics.checkParameterIsNotNull(x, "x"); if (x instanceof String) { int var1 = ((String)x).length(); System.out.println(var1); var1 = ((String)x).length(); System.out.println(var1); var1 = ((String)x).length(); System.out.println(var1); var1 = ((String)x).length(); System.out.println(var1); } } Kotlin Decompile 

੺ݎಞ 5. kapt ● Kotlin plugin supports annotation processors like Dagger or DBFlow. ● kotlinc ▶︎ Kotlin Stubs ▶︎ javac ▶︎ Final Code ● Clean Build ೧ঠ૑ ઁ؀۽ ز੘ೞӝب ೣ ● ঌ ࣻ হח ী۞ apply plugin: 'kotlin-kapt' 

੺ݎಞ 6. ੗߄৬ ৮੹ೠ ੉߹਷ ࠛоמ ● Framework Ú Java ● Library Ú Java / Kotlin ● Your Code Ú Java / Kotlin ● Kotlin Compilerо Java Class Codeܳ ݅ٞ ● Java to Kotlin, non-null ё୓ী null ਸ ࠁյ ҃਋ …. ী۞ ߊࢤ 

൞ݎಞ 

൞ݎಞ 1. Java ޙઁ ೧Ѿ Java 6 2006 Java 7 2011 Java 8 2014 Android 1.0 2008 Java 7 Support 2013 Java 8 Support Jack / desugar Different for every version Java 9 ???? 

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൞ݎಞ 2. Kotlin Android Extensions ● Kotlin Android Extensions ● View Binding, ViewHolder (1.1.4), Parcelable (1.1.4) ● Extension Function ● Lazy ● Anko 

൞ݎಞ 2. Kotlin Android Extensions import kotlinx.android.synthetic.main.activity_main.* class MyActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) textView.setText("Hello, world!") } } Kotlin Android Extensions 

൞ݎಞ 2. Kotlin Android Extensions apply plugin: 'kotlin-android-extensions' import kotlinx.android.synthetic.main..* // kotlinx.android.synthetic.main.activity_main.* 

൞ݎಞ 3. טযաח Kotlin GitHub ഛੋ (2017/09/20 ӝળ) ● Stars : 5ਤ ● Language : 30ਤ 

൞ݎಞ 3. טযաח Kotlin ● ࠶۽Ӓ / ߊ಴ / ҳӖ ࢠ೒ী Kotlin ࢎਊ੉ ૐо ● ӝઓ Java 100% ▶ Java (N %) + Kotlin (M %) ● ׮೯൤, ೠӖ۽ ੘ࢿغח Ӗٜ੉ ࢤӝҊ ੓਺ ● ٘٣য, ೠӖ۽ ػ ଼ 1ӂ੉ ୹द ● ழפ੄ ௏ౣܽ (୹द ৘੿) 

൞ݎಞ 4. Simple is Best ● Delegation ● Data Class ● Singleton ● Null Safety ● Expression ● Destructuring Declarations ● Type Checks and Casts 

val person = Person().apply { name = "Pluu" age = 1 money = 1 phone = "123-1234-1234" // More } ൞ݎಞ 5. Code Scope 

val person = Person().apply { name = "Pluu" age = 1 money = 1 phone = "123-1234-1234" // More } ൞ݎಞ 5. Code Scope apply Calls the specified function [block] with `this` value as its receiver and returns `this` value. 

val person = Person().apply { … } ൞ݎಞ 5. Code Scope 

൞ݎಞ 6. IntelliJ ● Show Kotlin Bytecode ▶ Decompile ● Decompile ػ Java ௏٘۽ ୓௼ оמ 

Preferred / Used Kotlin 

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Any.kt 

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http://tech.lezhin.com/2017/08/03/the-case-against-kotlin 

● 2017֙ ୡࠗఠ Kotlin ҕࠗ ౥౥൤ (पઁ ҕࠗ 3~4ѐਘ) ● GDG Kotlin झఠ٣ ଵৈ ● Kotlin ੗ܐ (Kotlin Reference, Twitter, Medium, Qiita) ● ࢎղ Kotlin झఠ٣ ૓೯ (ঌ۰઱Ҋ ղо ॶ۰Ҋ) ղо ೠ ೯ز 

૑Ә द੘೧ب ן૑ঋইਃ ● ੉ઁ ݆਷ ࢎۈٜীѱ ҙबਸ ߉ӝ द੘ ● ই૒ ੗ܐо ੸૑݅, Բળೞѱ Ӗٜ੉ ৢۄ১ ● ੗߄৬ э੉ ࢎਊ೧ب غח ੼ ● ௏٘о ब೒ೞѱ ੘ࢿ оמ೧૗ ● best practice ࠗ੤ ● ಩և਷ ٣੗ੋ ಁఢ ੸ਊ оמ 

● ן૑ ঋওযਃ ● Kotlin ೙ࣻ X, খਵ۽ Kotlin਷ ழ૕ ৘੿ ● Convert Java File to Kotlin File ۽ оߺѱ द੘ ● Show Kotlin Bytecode ۽ ઺р ୓௼ оמ ର੉ܳ ݅٘ח Action Item! 

ଵҊ ● Kotlin Reference (https://kotlinlang.org/docs/reference/) ● Get Started with Kotlin — It is here to stay (https://medium.com/peachstudio/get- started-with-kotlin-it-is-here-to-stay-1eaac85ae6a0) ● Kotlin: A New Hope in a Java 6 Wasteland (https://academy.realm.io/posts/ droidcon-michael-pardo-kotlin/) ● Kotlin੄ ࡄҗ Ӓܿ੗ (http://tech.lezhin.com/2017/08/03/the-case-against-kotlin) 

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